Overall, we aimed to discover more about mast cell physiology, focusing on their homeostatic regulation in vivo, their activation in vitro and in allergic disease, their gene expression patterns, and their surface antigens. In our first study, our objective was to establish the function of Src homology 2-containing inositol 5’-phosphatase (SHIPI), in mast cells in vivo. SHIP1 inhibits immune receptor signaling through
hydrolysis of the phosphatidylinositol-3 kinase (P13K) product Pl-3,4,5-P₃ ,forming P1-3,4-P₂. In mast cells, SHIPI represses FcεRI- and cytokine-mediated activation in vitro, but little is known regarding the function of SHIPI in mast cells in vivo, or the susceptibility of Shipl⁻/⁻ mice to mast cell-associated diseases. We found that ⁻ mice have systemic mast cell hyperplasia, increased serum levels of IL-6, TNF, and IL-5, and a heightened anaphylactic response. Further, by reconstituting mast cell-deficient mice with Ship1⁺/⁺ or Shipl⁻/⁻ mast cells, we found that the above defects were due to loss of SHIPI in mast cells. Additionally, we found that mice
reconstituted with Shipl⁻/⁻ mast cells suffered worse allergic asthma pathology than those reconstituted with Ship1⁺/⁺ mast cells. In summary, our data show that SHIPI represses allergic inflammation and mast cell hyperplasia in vivo, and that SHIPI exerts these effects specifically in mast cells. In our second study we compared Lin⁻Sca-1⁺c-kit⁺ (LSK) cells, which are highly
enriched for hematopoietic stem cells (HSC), and mast cells, using microarray expression analysis, and identified prion protein (PrPC) as a potentially novel marker of mast cells. Upon further investigation, we found that PrPC (1) is expressed on the surface of human and mouse mast cells, both in vitro and in vivo; (2) is not required for mast cell differentiation or tissue homeostasis; (3) is released by mast cells at
steady state and rapidly upon activation; and (4) is released in response to mast cell dependent allergic inflammation in vivo. Since mast cells are long-lived and known to traffic to the brain and central nervous system (CNS), our observations could have important implications for the transmission and pathology of prion diseases. Further, mast cells could be a unique system to investigate PrPc’s normal function.